Mechanical equipment used in the handling of liquids or gases may be subject to leakage problems, for example, valve stems, shafts or rods. The successful use of such equipment to contain and handle liquids or gases requires adequate control of this leakage, and several sealing methods and devices have been used to achieve such leakage control.
Compression packing is one of the most common devices used in sealing, and is used in many industries, including chemical, pharmaceutical, marine, sewage, and others. Compression packing involves the insertion of the packing made from soft, pliant materials into the space (i.e., the stuffing box) between a rotating or reciprocating member of a pump or valve and the body of the pump or valve. When pressure is transmitted to the packing materials, the materials expand against the stuffing box and the valve or pump member, thereby creating a seal. Compression may be applied to packing by means of packing bolts which are attached at one end to a clamp around the valve body and at the other end to a spigot, a flange or other projection bearing on, integral with or attached to the gland or sleeve which bears against the packing. Tightening of the packing bolts, therefore, increases the pressure on the packing and thereby exerts the radial pressure on the stem and the stuffing box. The resulting radial pressure of the packing onto the stem and stuffing box provides the desired seal so long as the radial pressure exceeds the pressure of fluid in the valve.
Improper loading is a condition wherein the sealing compression exerted by the gland follower on the packing is either insufficient or excessive. Packing volume variations and bolt creep are contributing elements of improper loading, because both will induce changes in the compressive force applied by the gland follower on the packing. But inaccurate torquing of the gland bolts by workers may also cause improper loading. Such inaccurate torquing may be the result of human errors. However it is recognized that even when torque wrenches are used by workers they are often inaccurate, resulting in improper loading. Leaks thus occur from the outset because the load on the packing is insufficient to achieve or maintain a seal, or excessive to damage the packing. Fluid leakage along the shaft of valves and pumps has long been recognized as a serious problem in power and industrial plants. In recognition of this problem, various attempts have been made to obtain leak free performance and reduce maintenance requirements for a pump or a valve. For example, improved packing materials have been developed for a larger range of temperatures, better chemical resistance and improved coefficient of expansion characteristics. Torque values have been established for the bolts connecting the gland follower to the stuffing box. Installers follow such specifications to apply a proper load to the packing to achieve a seal, but as discussed above may not attain a proper load. Several companies have initiated routine maintenance programs that include re-torquing of gland follower bolts. Such re-torquing is done frequently because of the significant risk posed by improperly loaded gland bolts and the resulting leakage of fluid from the apparatus.
Another attempt to obtain leak free performance and reduce maintenance requirements involves live-loading of the gland follower. Live-loading (or “dynamic loading”) refers to the mounting of compressed springs on the gland follower whereby a continuous force is exerted on the gland follower to insure a regular compressive pressure is exerted on the packing. Although coil springs could be used, it is conventional practice to use so-called Belleville springs which are essentially formed as a stacked series of dished washers that flatten when compressed. A significant amount of force is required for this compression. Such springs have higher compression rating than simple coil springs.
The use of Belleville springs provides a live-load system which can continuously compensate for changes that may take place in the packing under operating conditions of the valve, for example high pressures and temperatures. Polytetrafluoroethylene (PTFE) packings for instance, are very susceptible to undergo volume changes when exposed to temperature variations since the thermal expansion coefficient of PTFE is nearly ten times greater than that of steel. In such cases, the volume of the material may reduce under operating conditions and, whereas this could harmfully affect the sealing in an unsprung valve, the spring force will compensate for this reduction and maintain the packing under pressure. Alternatively, if the packing volume increases, the pressure on the stem, gland follower and stuffing box in an unsprung valve could increase too much and possibly cause sticking of the stem, extrusion of the packing or both. The live-loaded valve however can accommodate the pressure increase by means of further compression of the springs.
Thus, the live-loaded packing construction can provide a useful amount of self-adjustment, but the exact amount of force actually being exerted on the packing typically remains unknown. Accordingly, it is very difficult to precisely determine if the correct load is actually being applied to the compressible packing material.
Therefore, a need exists to provide an improved dynamically-loaded packing system that not only supplies the amount of self-adjustment necessary to maintain adequate pressure on the packing, but also indicates the force being exerted on the packing at all times to thereby prevent improper loading of the packing system. It is to this need that the present invention is primarily directed.